1. Field of the Invention
This invention relates to fan speed controls and, more particularly, to a linear actuator system for adjusting the setting of a fan speed control.
2. Description of the Related Art
Controlling the speed of a fan, of the type used for cooling, circulating, or exhausting air, is often desirable. Varying fan speed can accommodate changing conditions of temperature and humidity and can conserve energy. Fan motor controls have, for a long time, varied fan speed by using switches that connect various portions of the motor armature to a power source. This requires, however, that the motor armature have multiple windings and/or multiple taps, which increases the cost of the motor and decreases its reliability. The mechanism used to switch between the different windings is typically located on or near the motor housing. This can be inconvenient if, for instance, the fan is mounted on a high ceiling, out of reach. For these reasons, it is desirable to provide an external device, which is easily mounted on a wall, to regulate the speed of a fan motor.
Light dimmers have been adapted to control fan speed by controlling the flow of power from an ac source to the fan motor. See, for example, U.S. Pat. No. 4,520,306, issued May 28, 1985 to Kirby, incorporated herein by reference. Kirby discloses a phase control device that incorporates a power semiconductor switch, which alternately connects and disconnects power to the fan motor in each half cycle of power flow from the source. The fan speed is varied by controlling the period of time during which the motor is connected to the source. A drawback of these phase-control devices is that the semiconductor-switched waveform provided to the motor tends to produce audible high-frequency noise.
More recent fan speed controls use circuitry having no semiconductor switches. (See U.S. Pat. No. 4,408,150, issued Oct. 4, 1983 to Holston et al.) These "quiet" fan speed controls provide several selectable impedances - typically capacitors - in series with the motor armature to provide various reduced levels of smooth (i.e., not switched) power to the motor from an ac source. The power reduction is proportional to the series impedance. The De-Hummer Fan Speed Control, manufactured by Power Controls, San Antonio, Tex., is a wall-mountable device operated by a rotatable switch and actuator. The switch can be positioned to select among four capacitance values, providing four discrete speed settings and a power off setting.
A drawback of rotatable switches is that they do not provide a quickly-discernible indication of their setting, since the rotation has no inherent starting or ending point. A linearly actuable switch overcomes this drawback, since the relative speed setting is easily determined from the position of the actuator relative to its end of travel points.
Conventional linearly actuated switches are designed to be operated by one or two fingers moving relative to a hand that is fixed within a local frame of reference. They are not well adapted for wall-mounted use, where an entire arm may be used to actuate the switch. Specifically, the force required to move the switch from one position to the next typically results in overshoot, due to the feedback control characteristics of the human arm/muscle system, which quickly pushes the actuator past the desired position. This makes a wall-mounted linear switch difficult to use.
The Sunrise Whispurr fan speed control, manufactured by Lightolier Controls, Secaucus, N.J., includes a four-position linear slide switch for selecting among three speed settings and "off." The control requires a large actuation force (between about 1.5 and 2 lb.) to move the slide switch actuator from one position to the next. In addition, the actuator can be set between adjacent positions so that the switch becomes inoperative and power is removed from the fan motor.